1. Field of the Invention
This invention relates generally to the fully differential non-operational amplifier (non-op-amp) based unity gain filter. More particularly, this invention relates to a fully differential positive feedback bipolar junction transistor (BJT) biquad filter using cascode wide band unity gain buffer.
2. Description of the Prior Art
Because of recent progress made in optical integrated circuits (OICs) technology and optical communication system designs, wide band communication systems such as Fiber Distributed Data Interface (FDDI) and Synchronous Optical Network (SONET) impose greater demand on filter circuit design to achieve high bandwidth requirements in these systems, particularly filters utilized for front end analog applications. As these types of communications systems have data transmission rate ranging from 100 Mbits/sec to 200 Mbits/sec or even higher, the filters used for the analog front end must also achieve high speed operation. Therefore, the design specifications for these filters generally included performance requirements such as high speed, low power consumption, wide dynamic ranges, high noise rejection ratio, and greater scale of integration and packaging.
Most of the conventional types of filters are no longer able to satisfy these requirements. One common filter used in the prior art is a passive type of filters which generally include circuit elements of resistors, capacitors, and inductors, i.e., the R-L-C filters. This type of filters are no longer suitable for most modern electronic applications because of the disadvantages that these circuits have larger volume and that the inductance circuits are not suitable for design and fabrication on ICs with large scale of integration.
Another type of filters commonly used in the prior art is a operational amplifiers based (Op-Amp-based) active filter. This type of filters can be structured in many different forms to implement design factors such as negative or positive feedback, single ended or fully differential. The structures of this type of filters are altered to achieve various design purposes such as application of the filter as a unity gain follower, for increasing the noise rejection ratio, or for lowering the power consumption requirements. This type of filters are however not suitable for wide band communication application due to the speed limitation of the Op-Amp-based filters.
One specific prior art fully differential op-amp-based filter is a dual single-ended operational filter, or generally known as Allen-Key filter, which is used to achieve unity gain. However, this type of filters are not implemented due to the poor noise rejection and hence poor dynamic range.
Wang et al. discloses in U.S. Pat. No. 5,117,199 entitled `Fully Differential Follower Using Operational Amplifier` (issued on May 26, 1992) a fully differential amplifier with a unity gain. The amplifier includes a differential input stage having two pairs of differential inputs. One pair receives the input signal. The other pair is connected internally to receive differential mode feedback from the output stage.
These two pairs of differential inputs are combined with a common node feedback signal and cascoded to the output. The amplifier incorporates automatic internal noise cancellation due to its differential mode feedback. Construction of the circuits by the use of CMOS or BiCMOS technology is disclosed which may be applied for a higher frequency of operation up to 10 MHz. Due to the general characteristics of the op-amp-based unit gain filter, the technique disclosed by Wang et al. has a slower frequency response. Therefore, the achievable bandwidth is not sufficient for application to filters to be operated at higher frequency range which is now required in modern data processing, transmission and communication applications.
A non-op-amp based active filter is disclosed by Chung-Yu Wu et al. (`Design Techniques for High-Frequency CMOS Switched Capacitor filters Using Non-Op-Amp-Based Unity-Gain Amplifiers`, IEEE Journal of Solid State Circuits, Vol. 26, No. 10, October 1991). A fully differential non-op-amp-based unity gain amplifier is disclosed. The disclosed filter has a normal gain of unity but it has greater bandwidth, better settling behavior, smaller chip area, and less transistors and thus can be used to replace unity-gain buffers (UCBs) which is the amplifiers used in the switched-capacitor (SC) filters with high gain op-amps and unity-gain feedback. Furthermore, since the error caused by the nonlinear parasitic capacitance and process variations can be compensated by tuning the gain of the filter, this filter can be used in a switched-capacitor filters (SCFs) operated in a relatively higher frequency range with lower parasitic errors.
However, the filter as disclosed by Wu at al. has the limitations that the precision of the filter is affected by the mismatch between CMOS devices. Additionally, the speed of the filter is still limited by the parasitic capacitance when a CMOS technology is applied. The parasitic capacitance which exits generally between the source and the gate of a CMOS transistor typically is in the order of few pico-farad. Due to this parasitic capacitance, the filter cannot achieve a frequency level in the range over 100 MHz.
The techniques offered by the prior art are still limited by several major difficulties including the limitations of bandwidth and device mismatches. Furthermore, in order to achieve higher bandwidth operation, external devices are often employed which causes the difficulties in non-integrability and size limitation in integrated circuit (IC) implementation. These difficulties have caused adverse effects on system performance, power management, cost and reliability.
Therefore, there is still a need in the art of non-op-amp-based filters and their application in wide band communication system design to have a fully-differential filter which is suitable for high bandwidth application and which also has improved power supply rejection ratio and dynamic range.